Tankyrases-1 and -2 (TNKS-1 and -2) are members of the poly(ADP-ribose)polymerase (PARP) enzyme superfamily, which modify and regulate target proteins by addition of multiple (ADP-ribose) units from the substrate NAD+. TNKS-1 and -2 have many cellular roles, including regulation of elongation of telomeres, activation of nuclear mitotic apparatus protein (NuMA) in mitosis and regulation of the Wnt signalling pathway. This makes the tankyrases attractive new targets for design and development of new anti-cancer drugs.
2-(4-Trifluoromethylphenyl)-7,8-dihydro-3H-thiopyranopyrimidin-4-one (XAV939) was one of the few active inhibitors of tankyrases reported until 2013. The aim of this project was to explore the structure-activity relationships towards enhancing potency and selectivity by replacing the saturated sulfur-containing ring with saturated and unsaturated nitrogen heterocycles and by varying the aromatic side-chain.
Firstly, ascorbate-modified Sonogashira couplings of bromocyanopyridines and a variety of 4-substituted arylethynes, followed by acidic cyclisation and conversion of the lactone into the lactam, gave differently substituted arylnaphthyridinones. The alternative route used transition-metal-free reaction of bromopyridinecarboxylic acids with symmetrical β-diketones. 7-Phenyl-1,6-naphthyridin-5-one and 7-(4-methylphenyl)-1,6-naphthyridin-5-one were converted to the N1-oxides. Alkylation at 1-N gave 7-aryl-1-methyl-5-oxo-5,6-dihydro-1,6-naphthyridin-1-ium iodides and subsequent reduction gave saturated target 7-aryl-1-methyl-1,2,3,4-tetrahydro-1,6-naphthyridin-5-ones.
Other target compounds included pyridopyrimidinones, which were prepared from the corresponding bromopyridinecarboxylic acids by a copper-catalysed reaction with 4-substituted benzamidines. Target tetrahydropyridopyrimidinones, however, were obtained from condensation of 1-benzyl-4-oxopiperidine-3-carboxylic esters with substituted benzamidines.
All compounds were evaluated in vitro for inhibition of the catalytic activity of TNKS-2. The best compounds were investigated further, including in vitro TNKS-1 and PARP-1 inhibition and anti-proliferative studies on HT29 and FEK4 cell lines. Notably, 1-methyl-7-(4-trifluoromethylphenyl)-1,2,3,4-tetrahydro-1,6-naphthyridin-5-one and 1-methyl-7-(4-methoxyphenyl)-1,2,3,4-tetrahydro-1,6-naphthyridin-5-one showed 50% inhibition of TNKS-2 at 1.5 nM and 1.1 nM, respectively, showing also high selectivity (IC50 against PARP-1: 4.8 μM and 3.4 μM, respectively). This high potency and selectivity point to potential for development towards therapeutic use in cancer. A patent covering these discoveries has been filed.